Dimmer circuits are commonly used to control power, in particular alternating current (AC) mains power, to a load, such as a light source. In one existing method, a light source can be dimmed using phase controlled dimming whereby power provided to the load is controlled by varying the amount of time that a switch connecting the load to a mains power source is conducting during a cycle of the AC (i.e. varying the duty time). Specifically, AC power to the load is switched ON and OFF during each half cycle of alternating current and the amount of dimming of the load is provided by the amount of ON time in relation to the OFF time for each half cycle.
Phase control dimmer circuits generally operate as trailing edge or leading edge dimmer circuits, and the two circuits are suited to different applications. In leading edge circuits, power is switched OFF at the beginning of each half cycle. In trailing edge circuits, power is switched OFF later in each half cycle (e.g. towards the end of each half cycle). Leading edge dimmer circuits are generally better suited to controlling power to inductive loads, such as small fan motors and iron core low voltage lighting transformers. Trailing edge dimmer circuits, on the other hand, are generally better suited to controlling power to capacitive loads, such as drivers for Light Emitting Diode (LED) lights.
Phase control dimmer circuits generally switch ON and OFF AC power with a high voltage to the load. Generally, the dimmer circuit and the load are connected in series to the AC power. Thus, if a defect in the load circuitry or in the load itself occurs, the dimmer circuit will see a short-circuit as the load which can cause a sudden surge of high current which is damaging to the load and/or the dimmer circuit. Accordingly, exemplary prior art phase control dimmer circuits employ various techniques to provide short-circuit protection to guard against load faults, such as incorrect wiring of the load circuitry.
More specifically, in existing examples of MOSFET switched dimmer circuits, a short-circuit event or an over current condition can be determined by: monitoring voltage drop across series current sense resistor element, monitoring voltage drop across an intrinsic diode (in AC half-cycle polarity when intrinsic diode is forward-biased) of the MOSFET, or monitoring voltage drop across the MOSFET channel resistance (in AC half-cycle polarity when intrinsic diode is reverse-biased). In the example where the voltage drop across the MOSFET channel resistance is monitored, an additional comparator circuit required. It will be appreciated by those persons skilled in the art that the MOSFET channel resistance is a component of the the ON-state resistance of a MOSFET switched dimmer circuit. For example, the ON-state resistance of the MOSFET is 1Ω at the highest operating temperature.
In this example, the dimmer circuit is a trailing edge phase control dimmer circuit having a MOSFET switching circuit for controlling delivery of AC power to a load and a switching control circuit for controlling switching of the MOSFETs. The MOSFETs are configured so that they alternately control power delivery to the load over the different polarity half cycles of AC power. That is, the MOSFETs turn-ON and turn-OFF the switching circuit at each cycle of the AC, respectively, so that the load (e.g. a driver for LED down lights) is dimmed in proportion to the amount of time in each cycle that the switching circuit is switched OFF. The MOSFETs of the exemplary switching circuit have an ON-state (conducting state) resistance made up of several components of resistance including: MOSFET source diffusion resistance, channel resistance, accumulation resistance, “JFET” component resistance, drift region resistance, and substrate resistance. In the example, the additional comparator circuit is employed to compare the MOSFET ON-state voltage drop with a reference voltage to determine whether a short-circuit condition has occurred in the exemplary dimmer circuit and/or the load. If the MOSFET ON-state voltage drop is greater than the reference voltage then the comparator circuit activates a cut-out circuit to remove gate drive from the MOSFET.
It will be appreciated by those persons skilled in the art that the MOSFET ON-state voltage drop is indicative of the load current. An increase in magnitude of the load current is indicative of a short-circuit condition occurring and the gate voltage of the MOSFETs is subsequently modified to turn-OFF the MOSFETs. The comparator circuit, however, must have components selected to withstand the high voltage across the MOSFETs when it is switched OFF in the non-conducting state, which adds additional complexity and cost to the exemplary dimmer circuit.